Zethus was one of the twin sons of the god Zeus and the mortal Antiope in Greek mythology, who along with his brother Amphion was a protagonist in one of the founding myths of the city of Thebes. His mother was raped by Zeus, and fled her homeland; she went to Sicyon, where she married the king, Epopeus. A Theban general, however, attacked Sicyon and carried her back; on the way, Antiope gave birth to Zethus and Amphion and was forced to abandon them. In Thebes, the general gave her to his wife, who mistreated her for a long time afterwards.

Antiope managed to escape many years later, and searched for her children. She eventually found them and told them what had happened. The brothers killed the general's wife, and after forming a small army, attacked Thebes and took the throne. Zethus became a skilled hunter and herdsman, while his brother became an expert at playing the lyre. Together, they constructed the walls of Thebes. Zethus then married Thebe, whose name was later given to the city. They had a son, who died after Thebe made a mistake, and Zethus committed suicide out of grief.